The features contributing to the reduced or enhanced antineoplastic activity of alpha-N-heterocyclic thiosemicarbazones will be determined by comparing the structural features of several derivatives. Complete three-dimensional, single-crystal X-ray diffraction data will be collected for isomers of substituted 2-formylpyridine thiosemicarbazones, their iron chelates, and a closely related isoquinoline iron chelate. The intensity data will be reduced to structure amplitudes, the free thiosemicarbazone derivative structures solved by statistical methods, and the iron complex structures solved by heavy atom methods. The coordinates and thermal parameters for all atoms will be refined by the method of least-squares and analyzed both numerically and graphically. The results of this study will answer questions about the bonding and conformational parameters necessary for antineoplastic activity and the bulk tolerance and types of groups adjacent to or at the active center of the enzyme ribonucleoside diphosphate reductase so that new derivatives with enhanced antineoplastic activity can be synthesized.